Chemical sensing using field effect transistors (FETs), for example, a FET capable of detecting gas as described in the 1975 article I. Lundström, “A Hydrogen-Sensitive MOS Field-Effect Transistor” Applied Physics Letters 26, 55-57 and in U.S. Pat. No. 4,058,368 awarded to Svensson et. al. In the aforementioned article the transistor device has a palladium gate that is catalytically active with hydrogen dissociatively adsorbing and atomic hydrogen then absorbing into the palladium gate.
Chemically sensitive FETs with an air gap between the channel and the gate have been described in e.g. Janata's 1985 U.S. Pat. Nos. 4,514,263 and 4,411,741 which describe a chemically sensitive field effect transistor with an air gap for measuring components in either gases or liquids. These FETs are similar to the Lundström MOSFET in that they both include a doped semiconductor substrate which acts as a channel and a pair of doped regions forming source and drain electrodes, however in Janata's device the metal gate is suspended above the semiconductor substrate, defining an air gap. A voltage is applied to the gate and when gases or liquids with a dipole moment are introduced into the air gap, they are attracted to the charged gate or to the semiconductor surface.
When chemically sensitive FETs are used in chemical sensing applications, that is, when such a FET is used in/as a sensor, the sensor further comprises means for electrically controlling and/or biasing the FET for sensing so that detection of the chemical will manifest as or in an electrical signal, which thus becomes a chemical indicative electrical signal. By predetermining the relation between the sensed chemical, e.g the concentration thereof, and the resulting chemical indicative electrical signal, the electrical signal can be used solely, not only to detect presence or not of the chemical, but also to determine the amount, typically the concentration, of the chemical.
Following the isolation of graphene in 2004 by K. S. Novoselov et al, “Electric Field Effect in Atomically Thin Carbon Films” Science, 306, 5696, 666-669, its use in various application areas has been investigated, including for chemical sensing applications. Lower detection limits have become possible, in part due to the low noise of graphene transistors and also due to the high surface area and the large field effect in graphene which enables large shifts in the Fermi level by applying a gate voltage, thus tuning the electronic properties for sensor applications.
Parts-per-billion detection was demonstrated in the 2007 article Schedin et. al., “Detection of individual gas molecules adsorbed on graphene”, Nature Materials, 6, 9, 652-655, and showed stepped noise indicating individual molecular adsorption and desorption events. In the aforementioned article, the graphene was in the form of small cleaved flakes.
The detailed mechanism behind the detection in graphene sensors is still a hotly debated topic, however, there is no doubt that graphene can be used in chemical sensing applications and that it enables lower detection limits than in conventional chemical sensing.
In Z. Cheng et. al., Nano Letters 10, 1864 (2010), a suspended graphene sensor is disclosed. It comprises a graphene field effect transistor (Gra-FET) fabricated from a mechanically exfoliated graphene supported on a silicon/silicondioxide substrate. Source-drain contacts of Cr/Au is defined by e-beam lithography and subsequent metallization. A polydimethylsiloxane (PDMS) chamber is incorporated over the “Gra-FET” chip to confine an electrolytic solution and a non leak Ag/AgCl reference electrode is used as an electrolyte gate. Etching of the silicon oxide underneath the graphene is carried out in situ to accomplish the suspension and make comparison possible between a situation with and without suspension, showing that suspension improved the sensing properties. However, the sensor is not practical, accomplishing suspension by in situ-etching is cumbersome and is prone to result in differences in the suspension between individual sensors, risk of damage to the graphene layer during the etching and risk of damage to the graphene layer when the substrate has been etched away under it.
A general problem with this device, and often a general problem with many disclosed devices involving graphene at the present date or at present, is that they are mainly designed for experimental purposes and not to be realizable as commercial products for practical use. They therefore, more often than not, have problems in areas such as robustness, reproducibility, individual variability, production yield and cost efficiency. For example, when low detections limits are sought, it does not matter if one individual sensor can be made very sensitive if this is not repeatable so that many sensors with, in principle, equal sensitivity and low detection limit can be manufactured, or, if the sensor, during use or manufacture, is likely to or too easily may have its sensibility negatively affected. Damage to the graphene layer may destroy or at least impair the chemical sensing capabilities. Since the degree and impact of damages typically are, more or less, random in nature, also small damages may contribute to undesirable individual variability among sensors of the same type and construction, which may be a particular problem for sensors with low detection limits, as enabled by graphene. It is therefore desirable to reduce the risk of damage to the graphene layer to the greatest extent possible.
A step toward practical gas sensing involving graphene was taken in the 2011 article G Lu et. al., “Toward practical gas sensing with highly reduced graphene oxide: A new signal processing method to circumvent run-to-run and device-to-device variations”, ACS Nano, in press/published online. The article discusses fabrication and characterization of gas sensors using a back-gated FET platform with chemically reduced graphene oxide as the conducting channel, and signal processing method that addresses device-to-device variations. The gate is a silicon wafer and on top of the gate there is a silicondioxide layer onto which Cr/Au electrodes were fabricated using e-beam lithography. A few drops of reduced graphene oxide suspension were cast onto the fabricated electrodes to accomplish a network of suspended reduced graphene oxide platelets left on the electrodes after solvent evaporation. The network serves as the conducting channel between the drain and source electrodes. However, the presented solution results in considerable differences in sensor response between individual devices with uncertainty of placement and thickness of the reduced graphene oxide platelets. Also, the flexible nature of graphene makes suspension of an entire flake unlikely and thus contribution from the SiO2 substrate highly probable.